The present invention relates generally to reaction of feed gas mixtures in regenerative heat transfer incinerators or oxidizers. More particularly, the present invention relates to apparatus and methods for oxidation of combustible components of feed gas mixtures in regenerative thermal or catalytic oxidizers. The present invention may be used to abate, through oxidation, combustible contaminants (e.g., volatile organic compounds, carbon monoxide, etc.) contained in gaseous industrial emissions.
Heat regenerative oxidizers are widely used to control air pollution from industrial sources. Regenerative oxidizers are characterized by heat sinks (i.e., beds of solid heat exchange material), which extract and store heat from the reacted feed gas mixture so that this heat may be used to increase the temperature of the incoming gas and thereby reduce external energy requirements of the system. These systems are configured in a variety of ways and include both regenerative thermal oxidizers (RTOs) and regenerative catalytic oxidizers (RCOs). Examples of heat regenerative oxidizers suitable for air pollution control service are shown and described in U.S. Pat. No. 5,823,770 (Matros, et al.), U.S. Pat. No. 5,366,708 (Matros, et al.), U.S. Pat. No. 5,364,259 (Matros, et al.), U.S. Pat. No. 5,163,829 (Wildenberg) and U.S. Pat. No. 5,161,968 (Nutcher, et al.).
The gaseous emissions treated in RCOs or RTOs often have a relatively low concentration of volatile organic compounds (VOCs) and other combustible contaminants. The adiabatic temperature rise of VOC oxidation in heat regenerative systems is often below 30.degree. C. Under these conditions, heat regenerative systems typically require supplementary heat input in order to maintain the temperature in the combustion chamber of an RTO or catalytic zone of an RCO sufficiently high to achieve the desired degree of oxidation of contaminants in the gas to be treated. Conventionally, externally fueled burners and, to a much lesser extent, electric heating elements are employed as heaters to introduce supplemental heat energy into the system. A typical RTO installation includes one or more burners directed into the combustion chamber. Likewise, a typical dual bed RCO includes one or more burners directed into the duct through which the gas passes from one catalytic zone to the other. In addition to providing supplemental heat to the system during operation, the supplemental heat source is used to initially heat the system at startup.
After startup, the heater is commonly operated using continuous modulated control. For example, in the case of a burner, the rate at which fuel is delivered to the burner (i.e., the fuel load) is regulated by a controller which turns a valve in the fuel supply line in response to changes in one or more measured control variables, such as the temperature measured inside the combustion chamber or within the catalyst or beds of heat exchange material. Three-mode proportional-integral-derivative controllers are usually employed. The objective of conventional control schemes is to deliver enough fuel to the burner to maintain the measured temperature essentially constant at the established set temperature value. Overshooting the set temperature to any significant degree and off-set are purposely avoided by the control algorithm which may be quite complicated and include such factors as the magnitude of the difference between the measured temperature and the set temperature and the rate of change of the measured temperature. Thus, rather than simply decreasing or increasing the fuel load to the burner when the measured temperature is above or below the set temperature value, the control response varies throughout the range permitted by the valve as needed to avoid significant excursions from the set temperature. In practice, conventional control results in a moderate fuel load being delivered to the burner throughout much of the process. Likewise, conventional control of electric heaters used to provide supplemental heat energy to a regenerative heat transfer oxidizer system typically results in continuous, moderate power input to the heater.
In order for regenerative oxidizers to function efficiently, it is desirable to maintain the temperature profile across those regions of the system in which VOC oxidation primarily occurs (i.e., within and adjacent the combustion chamber in RTOs and within and adjacent the catalytic zones of RCOs) substantially uniform and at or just above the minimum necessary to achieve the desired VOC destruction efficiency. Nevertheless, many existing regenerative oxidizer systems are plagued by substantial temperature variance across these critical regions of the system. Oxidation of VOCs is compromised in the fraction of the gas flow not heated to the requisite minimum temperature, ultimately reducing the overall VOC destruction efficiency of the system. Although the input load to the supplemental heat source may be increased generally in an attempt to raise the temperature profile across the entire system and remove "cold spots", this practice results in increased fuel costs and potential overheating and decreased service life for process equipment, heat exchange material and catalyst. U.S. Pat. No. 4,877,592 (Matros, et al.) discloses a method of catalytic cleaning of exhaust gases in an RCO in which nonuniform temperature profiles in the catalyst are reduced by stirring the gas exiting a first catalytic zone before being introduced into a second catalytic zone. The gas is stirred using a fan, double-segmented grid or mixing tube. This proposed solution suffers from increased system complexity and equipment costs.